A New Model Yields a Better Picture of Methane Fluxes

Scientists update an old model with recent findings, allowing for a more accurate understanding of methane dynamics in wetlands.

Source:
Global Biogeochemical Cycles

Mayberry wetland, in California's Sacramento–San Joaquin River Delta. This peat-forming wetland is an example of an ecosystem with high methane emissions under inundated conditions. A new study seeks to clarify how methane dynamics change when the water table drops. Credit: Gavin McNicol

Peat-forming wetlands, including bogs and fens, can switch between acting as sources and sinks of methane, a highly potent greenhouse gas. Which process wins out depends on a multitude of factors, including climate, vegetation type, water table levels, nutrient inputs, microbe communities, hydrology, and the day-to-day conditions of the ecosystem. Current models can approximate net methane emission in these areas, but clearer predictions in the face of a changing climate require a more detailed model.

Methane is created when simple carbon-containing molecules, such as carbon dioxide and acetate, are reduced in the soil; that is, the carbon gains electrons and attracts neighboring hydrogen ions to form methane. Methane is destroyed when its carbon is oxidized, losing its electrons to nearby oxygen molecules. Both processes are mediated by soil microbes.

The formation of methane in soil occurs in highly reducing environments with no oxygen. In contrast, the reaction that destroys methane requires an environment where oxygen is present. The latter reaction is limited by the amount of methane and oxygen available.

This means that, generally speaking, methane is produced below the water table, where there is little to no oxygen, and it is destroyed above the water table, especially right at the boundary, where the most methane accumulates. When the water table is high, a greater proportion of the soil falls into methane-producing conditions. Likewise, when the water table drops, more soil is exposed to oxygen and thereby able to destroy methane. Current models commonly use this relationship to predict net methane production essentially on the basis of water table height.

Peatlands can switch from methane sources to methane sinks when the water table drops, but recent studies suggest that the controls on methane dynamics are not so simple. A new conceptual model, tested at this drained peatland pasture on Sherman Island, Calif., takes into account heterogeneity in methane dynamics through the vertical soil profile to provide a more accurate understanding of net methane emissions. Credit: Wendy Yang

Now Yang et al. suggest updating this classical conceptual model to include new information on methane dynamics gleaned from recent studies: For example, oxygen-poor pockets within the soil produce methane even above the water table, and methane can be destroyed below the water table in the absence of oxygen, depending on the presence of specific microbes and molecules in the soil that can play the role of oxygen to gain the electrons lost by methane.

The researchers proposed a new, heterogeneous conceptual model that takes into account the intricacies of methane dynamics for a more accurate picture of net methane emissions. They tested their model on a drained peatland in northern California, considering both the vertical soil profile and topography across the landscape. They found that although water table level can give a good general picture of methane emissions, the most accurate predictor of net methane production includes the abundance of oxygen-poor pockets.

This updated model provides researchers with a better tool for predicting methane emissions and feedback loops in relation to climate change. As our world continues to change, updating models to incorporate all the knowledge available will help scientists predict how the future, and the changes we make to our environment, will play out. (Global Biogeochemical Cycles, https://doi.org/10.1002/2017GB005622, 2017)

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